The present invention relates to a method for measuring the dynamical behavior of an elongate body which rotates about an axis. A contactless capacitive distance measuring system is used in which at least two measuring probes are arranged in such a manner that they pick up individual capacitance values representing the distance between a respective measuring electrode and the body. The body itself constitutes a total capacitance with respect to its environment. The invention also relates to a circuit arrangement for implementing the method.
In particular, the invention relates to a method of continuously and without contact measuring the distance between a probe and a galvanically isolated, i.e. conductively insulated, almost non-conductive rotating body.
To measure, for example, fast running gas ultracentrifuges with regard to dynamical and mechanical aspects in a vacuum, it is necessary to detect the movement of the long rotor simultaneously in several planes. The number of planes may vary e.g. from 10 up to 30.
Because the time relation between these distance signals is of great significance, all measuring systems employed must operate simultaneously and continuously. However, if a centrifuge is the object being measured, there arises the additional difficulty that the major part of the rotor may be composed of a non-conductive material, during operation all components (including the metal parts) are not conductively connected to ground potential and coupling with ground potential can be realized only by way of the capacitance formed between the rotor and the vacuum vessel. Thus it cannot be avoided that the object being measured causes the measuring signals to be mutually influenced.
Measurements at almost non-conductive bodies can be made at justifiable expense only with systems operating on a capacitive basis, particularly if the object is disposed in a vacuum. Capacitive distance measuring systems made, for example, by DISA Elektronik A/S (DK 2740 Skovlunde, Danmark) are available for this purpose and these have been used with success at the start of development of the new centrifuge generation.
In these systems, a change in distance is detected as a change in capacitance and is fed in the form of a frequency modulation- because the "distance capacitance" of interest is a component of an oscillator resonant circuit--to a suitable evaluation circuit. The system include oscillators which operate in a frequency range of 4 to 6 MHz. However, the actual frequency is now determined not only exclusively by the momentarily measured distance but also by many superposed parasitic influences, such as the configuration of the measuring probe, component scattering in the oscillators, length and position of the connecting cables between measuring probe/tuning plug/oscillator and the ambient temperature.
As long as only one measuring system is employed per object to be measured, all of these possible influences have only a slight effect on the accuracy of the measuring result, particularly if the measurements are short-time measurements and individual calibration is effected each time immediately before the actual measurement.
If a plurality of systems operate on an object to be measured and which is not conductively connected, but only capacitively coupled, all oscillators influence one another over the common rotor/vacuum vessel coupling capacitance. If oscillators operate at frequencies whose mixed products fall into the transmission bandwidth of the demodulators employed, the latter produce output signals which do not originate at all from the object being measured. Erroneous information can therefore not generally be excluded in conventional systems.
Additionally, the long-term stability of the available systems is so poor that frequency recalibration is necessary and this is associated with a great amount of time and, if done on running centrifuges, also with great risk.